One of the main advantages cited for optical signal processing is the capability of all-optical devices to switch in times much shorter than those required for electronics. In order to implement such devices, however, optical materials with suitable properties are required. In particular, it is necessary to be able to obtain a significant phase shift without suffering unacceptably high signal loss. As a rule of thumb, an acceptable (non-linear) optical material is one that can yield, due to the interaction between two optical pulses, a 180.degree. (.pi.) phase shift in less than the absorption length, i.e., the length over which the signal strength decreases by a factor e.sup.-1.
Switching means of the type relevant herein will be collectively referred to as "gates". For examples of all-optical gates see A. Lattes et al., IEEE Journal of Quantum Electronics, Vol. QE-19(11), p. 1718; D. Hulin et al., Applied Physics Letters, Vol. 49(13), p. 749; S. R. Friberg et al., Optics Letters, Vol. 13(10), p. 904; J. S. Aitchison, Optics Letters, Vol. 15(9), p. 471; E. M. de Sterke et al., Optics Letters, Vol. 14(16), p. 871; and M. N. Islam et al., Optics Letters, Vol. 16(7), p. 484; all incorporated herein by reference.
Silica-based optical fibers can meet the above rule of thumb criterion, even though their nonlinearity (expressed, for instance, in terms of the third-order susceptibility .chi..sup.(3) or the non-linear refractive index n.sub.2, which arises from the real part of .chi..sup.(3)), is very small, exemplarily about 3.2.times.10.sup.-16 cm.sup.2 /W. However, since the absorption in SiO.sub.2 -based fiber can be very small indeed (e.g., about 0.2 db/km at 1.55 .mu.m), it is possible to obtain the required phase shift without suffering unacceptable signal loss by providing a very long interaction length. See, for instance, M. N. Islam, Optics Letters, Vol. 16(7), p. 484.
Even though optical fiber can meet the above rule of thumb criterion, in many important potential applications of all-optical gates the use of optical fiber gates is at best inconvenient and may even be impossible, due to the relatively long length of fiber that is required. It thus would be highly desirable to have available a material that can be used to make an all-optical gate that involves a relatively short (of order 1 cm) interaction length L and can meet or surpass the above rule-of-thumb criterion. It would be especially desirable if the material were easily obtainable and had a mature fabrication technology such that components of the requisite small cross-sectional dimensions (frequently at least one of the dimensions is of order 1 .mu.m) can be easily fabricated, and if the low-loss wavelength region included the wavelengths that are of particular interest for optical communications, namely about 1.3 and 1.55 .mu.m. It would also be desirable if the material were compatible with photonic integrated device technology. This application discloses such a material.
It has long been known that the non-linearity in semiconductors can be orders of magnitude larger than that of silica-based fiber. The largest non-linearity typically is observed for wavelengths close to but longer than the wavelength that corresponds to the bandgap energy (E.sub.g) of the semiconductor. Unfortunately, at these wavelengths the .pi.-phase shift criterion typically can not be met without significant degradation of time response and loss, due to carrier generation. For instance, K. K. Anderson et al. (Applied Physics Letters, Vol. 56(19), p. 1834) report on measurements on AlGaAs ridge waveguides in the range 780-900 nm that found the linear absorption coefficient to be 16 cm.sup.-1 at 810 nm and 2 cm.sup.-1 at 830 nm. These wavelengths correspond to detuning of 20 and 40 nm, respectively, from the band edge of the material. These authors state that " . . . it is well known that although there is a resonant enhancement of n.sub.2 at wavelengths near the band edge, the increasing linear loss degrades the figure of merit for switching devices. Studies indicate that saturation of the signal intensity due to two-photon absorption limits the usefulness of the non-linearity for optical switching."
J. S. Aitchison et al. (Applied Physics Letters, Vol. 56(14), p. 1305) made measurements on GaAs/AlGaAs waveguides at 1.06 .mu.m and appear to suggest that ZnS might be preferable to GaAs for use in all-optical gates.
V. Mizrahi et al., (Optics Letters, Vol. 14(20), p. 1140) derive a criterion [(2.beta..lambda./n.sub.2)&lt;1; .beta. is the two-photon absorption coefficient, and &lt; is the vacuum wavelength of the radiation] that, according to these authors, should be considered in evaluating a material (e.g., organics) for all-optical switching. They also conclude that strong two-photon absorption (TPA), which may accompany a large n.sub.2, can severely hamper all-optical switching in any material.
M. Sheik-Bahae et al. (Physical Review Letters, Vol. 65(1), p. 96) show data on several semiconductors (no III/V semiconductors were included; the wavelengths were 0.532 .mu.m, 1.064 .mu.m and 10.6 .mu.m) that indicate that n.sub.2 (due to two-photon absorption) has a maximum at or near a wavelength that corresponds to E.sub.g /2.
K. W. DeLong et al. (Applied Physics Letters, Vol. 57(20), p. 2063) conclude, based on calculations that use the formulae of the above cited Sheik-Bahae paper, that " . . . in order to use the non-band-gap resonant nonlinearity in a semiconductor, the photon energy must be kept out of the regime where TPA is allowed . . . " (i.e. hv&lt;E.sub.g /2, where h is Planck's constant and v is the frequency), and refer to this as a " . . . very restrictive . . . " requirement.
Finally, K. Fujii et al. (Physical Review Letters, Vol. 65(14), p. 1808) have measured two-photon absorption spectra of quantum well structures in static electric fields for photon energies close to half the bandgap energy. The samples consisted of an undoped GaAs/Al.sub.0.4 Ga.sub.0.6 As multiple-quantum-well core region embedded in Al.sub.0.4 Ga.sub.0.6 As cladding regions. Measurements were made at wavelengths of, exemplarily, 1.561, 1.685 and 1.711 .mu.m.
Next will be discussed the background for a particular all-optical gate, namely, the gate disclosed in the parent of this CIP application, which application is in its entirety incorporated herein by reference.
In optical switching and transmission systems, it is important to periodically restore the logic level and timing of pulses traveling in the optical transmission medium. Such restoration is currently performed in regenerators, which typically include electro-optical devices. The current trend toward all optical systems has resulted in development of erbium-doped fiber amplifiers, which when used with soliton pulses, correct the pulse amplitude and shape and thus provide logic level restoration without the need for optical to electrical conversion. When such amplifiers are used, timing restoration is still needed since without such restoration the transmission or switching system can become limited by timing jitter and fluctuations (e.g. from background spontaneous emission, temperature variations, etc.). To date, an optical device for performing such restoration has not been available. However, other advances in soliton transmission and switching systems, such as the ultra-fast optical logic devices, described in U.S. Pat. No. 4,932,739, and the all-optical time domain chirp switch described in a copending application Ser. No. 07/609958 filed on Nov. 6, 1990, and assigned to the same assignee as the present application, are available for use in helping to address the problem described above.